Modeling a Space Battleship Yamato
I'd suggest to limit the numbers of vertical loop cuts to the same number of vertical lines along side the Yamato hull. 

Start with a low poly general shape of the main hull. 
Then start to split / separate the parts as new objects.
Parent everything to the main hull
Hide the main hull and start to develop the other objects into their proper shape.
Some details will be external objects placed along side the hull so don't worry in attempting to model the whole ship from a single mesh.

For the Wave Motion Gun you can use a small cylinder and then Joim the mesh with the main hull, bridging edge loops and creating faces when necessary